This invention relates to the catalytic treatment of hydrocarbons in the presence of hydrogen and under conditions of elevated temperature and pressure. More particularly, it relates to treating waxy paraffinic hydrocarbons, particularly full boiling range shale oils, so as to simultaneously lower the pour point thereof by catalytic hydrodewaxing and lower the organosulfur and/or organonitrogen contents thereof by catalytic hydrotreating.
Many hydrocarbon liquid feedstocks have the undesirable properties of high pour point, which causes pumping difficulties under low temperature conditions, and high organonitrogen and/or organosulfur contents, which are undesirable from the standpoint that such components deactivate certain refining catalysts or, if present in the ultimate product when combusted, contribute to atmospheric pollution. One such feedstock is raw shale oil, a feedstock obtained by retorting oil shale, such as the oil shale found in the Colorado River formation in the western United States. When retorted under temperature conditions above about 900.degree. F., a material in the oil shale known as kerogen decomposes, releasing shale oil vapors, which are condensed and collected by known techniques to produce raw liquid shale oil. Such raw shale oil is undesirable because it usually contains solid particulates, arsenic, and organonitrogen and/or organosulfur components. In addition, the raw shale oil has a high pour point, usually in the range of 50.degree. to 90.degree. F., indicative of the presence of a relatively high proportion of wax components, i.e., straight chain and slightly branched paraffins of high molecular weight.
Raw shale oil may be treated by known techniques to reduce the ash and arsenic contents thereof, and it is now known by the teachings in U.S. Pat. No. 4,153,540 issued to Gorring et al. that shale oil can be upgraded by a two-step method in which the shale oil is first contacted with a hydrotreating catalyst under conditions such that the organosulfur and organonitrogen contents of the shale oil are reduced. Subsequently, the hydrotreated shale oil is contacted with a hydrodewaxing catalyst under conditions (750.degree. to 1000.degree. F., 500 to 1500 psig, 0.25 to 1.0 LHSV, and a hydrogen feed rate of 5 to 6 moles per mole of feedstock) such that the feedstock is hydrodewaxed while its 750.degree. F.+ fraction is converted by at least 50% to products boiling below 750.degree. F. The hydrodewaxing catalyst employed by Gorring et al. is similar to that of Chen et al. described in U.S. Pat. No. Re. 28,398, that is, it comprises a ZSM-5 zeolite in its hydrogen form combined with a metal having activity for promoting hydrogenation/dehydrogenation reactions.
Although the two-step process described in U.S. Pat. No. 4,153,540 results in a significant reduction in the pour point of the shale oil, it also results in a shale oil product that contains undesirable proportions of organosulfur and organonitrogen components. In particular, the shale oil products reported in the Examples of U.S. Pat. No. 4,153,540 contain excessively high proportions of total nitrogen. One product, for example, contained 1.10 wt. % total nitrogen, representing only about a 50% reduction in organonitrogen components after two hydroprocessing steps. By most refining standards, such a shale oil product would require yet more hydrotreating to reduce the nitrogen content still further, for example, to below about 250 wppm.
In addition, the hydrodewaxing catalyst described in U.S. Pat. No. 4,153,540 exhibits an undesirable amount of hydrocracking. Ideally, one would want to treat the shale oil so as to substantially reduce its organosulfur and organonitrogen contents and its pour point without also (as would be the case in severe hydrocracking) substantially altering the boiling characteristics of the shale oil. But in the process described in U.S. Pat. No. 4,153,540, it appears that the hydrocracking is indiscriminate, that is, the waxy paraffins are hydrocracked sufficiently to lower the pour point but not without also cracking 50% or more of the 750.degree. F.+ components as well. Such excessive hydrocracking is especially undesirable if the shale oil is treated for pour point reduction at a distance remote from an oil refinery; it forces one to employ extensive recovery facilities for handling light end materials such as propane and butane and for generating hydrogen in a location where such is usually impractical. Additionally, and perhaps more importantly, to hydrocrack a shale oil so as to convert 50% or more of the 750.degree. F.+ fraction while only about 50% of the nitrogen is removed is an inefficient use of hydrogen, particularly when severe hydrocracking is not desired but low nitrogen-containing shale oil products are.
Accordingly, it is one of the objects of the present invention to provide a process for substantially reducing the pour point, sulfur content, and nitrogen content of shale oil feedstocks and other waxy hydrocarbon feedstocks while minimizing the amount of hydrogen consumed. It is another object to provide such a process having the further advantage of selectively hydrocracking the waxy paraffins while not substantially hydrocracking other components. It is yet another object to provide a process wherein a shale oil feedstock containing more than about 0.2 wt. % organosulfur components, and more than about 1.50 wt. % organonitrogen components, and having a pour point above about 50.degree. F. is converted, without substantially disturbing the boiling characteristics of the shale oil, to a shale oil product having a pour point below about 30.degree. F. and containing less than about 400 wppm organo-nitrogen compounds and less than 25 wppm organnosulfur compounds. It is yet another object to provide a catalyst having high activity for selectively hydrocracking a waxy, sulfur-containing and nitrogen-containing hydrocarbon feedstock such that a substantial reduction in the wax content thereof, as evidenced by a substantial reduction in pour point, is obtained simultaneously with a substantial reduction in the organosulfur and organonitrogen compounds contents. These and other advantages will become more apparent in view of the following description of the invention.